U.S. patent number 7,388,665 [Application Number 11/436,974] was granted by the patent office on 2008-06-17 for multicolour chromaticity sensor.
This patent grant is currently assigned to TIR Technology LP. Invention is credited to Ian Ashdown.
United States Patent |
7,388,665 |
Ashdown |
June 17, 2008 |
Multicolour chromaticity sensor
Abstract
The present invention provides an optical sensor having one or
more filter-photodetector pairs and feedback to monitor the
intensity and chromaticity of the white light generated by an
illumination system. According to the present invention,
filter-photodetectors are configured into pairs thereof, wherein a
first filter-photodetector of a pair is configured and arranged so
as to be sensitive to a predetermined region of the electromagnetic
spectrum, while a second filter-photodetector of the pair is
configured and arranged to be sensitive to a substantially
complementary region of the electromagnetic spectrum. The spectral
responsivity of the first filter-photodetector and the second
filter-photodetector overlap in a predetermined region of the
electromagnetic spectrum. Furthermore, the spectral responsivity of
the first filter-photodetector is configured to substantially
monotonically increase with respect to wavelength within said
predetermined region, while the spectral responsivity of the second
filter-photodetector is configured to substantially monotonically
decrease with respect to wavelength within said predetermined
region.
Inventors: |
Ashdown; Ian (West Vancouver,
CA) |
Assignee: |
TIR Technology LP (Burnaby,
CA)
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Family
ID: |
37430907 |
Appl.
No.: |
11/436,974 |
Filed: |
May 19, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070051881 A1 |
Mar 8, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60683436 |
May 20, 2005 |
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Current U.S.
Class: |
356/419; 250/226;
356/420 |
Current CPC
Class: |
G01J
3/10 (20130101); G01J 3/51 (20130101); G01J
3/513 (20130101) |
Current International
Class: |
G01N
21/25 (20060101); G01J 3/50 (20060101) |
Field of
Search: |
;356/419,420 ;250/226
;362/276 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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9836252 |
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Aug 1998 |
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WO |
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WO 2006122425 |
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Nov 2006 |
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WO |
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Primary Examiner: Punnoose; Roy M.
Attorney, Agent or Firm: Mayer Brown LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent
Application No. 60/683,436, filed May 20, 2005, which is
incorporated herein, in its entirety, by reference.
Claims
What is claimed is:
1. A multicolour chromaticity sensor enabling determination of
intensity and chromaticity of light, the sensor comprising: a) two
or more photodetectors, each having a detection zone responsive to
a range of wavelengths in the electromagnetic spectrum, wherein
each photodetector generates a sensor parameter representative of
intensity and chromaticity of light incident upon its detection
zone; b) two or more filters, one filter optically coupled to a
first of the two or more photodetectors thereby forming a first
filter-photodetector and a second filter optically coupled to a
second of the two or more photodetectors thereby forming a second
filter-photodetector; and c) the first filter-photodetector
configured to be sensitive to a first predetermined region of the
electromagnetic spectrum, and the second filter-photodetector
configured to be sensitive to a second predetermined region of the
electromagnetic spectrum, said first filter-photodetector and
second filter-photodetector forming a complementary pair, wherein
the first predetermined region is complementary to the second
predetermined region; wherein a measuring means interfaced with
each of the two or more photodetectors independently receives a
first sensor parameter from for one of the two or more
photodetectors and a second sensor parameter for a second of the
two or more photodetectors, thereby providing a means to determine
the intensity and chromaticity of the light.
2. The sensor according to claim 1, wherein the first predetermined
region and the second predetermined region are configured to
overlap in a defined zone of the electromagnetic spectrum.
3. The sensor according to claim 2, wherein spectral responsivity
of the first filter-photodetector monotonically increases with
respect to wavelength within the defined zone and spectral
responsivity of the second filter-photodetector monotonically
decreases with respect to wavelength within the defined zone.
4. The sensor according to claim 1, wherein the two or more filters
are selected from the group comprising thin-film interference
filters, transmissive dyed colour filters, filters having photonic
crystals with resonance abnormalities, guided-mode resonance
filters, tunable liquid crystal Lyot band pass filters and plasmon
filters.
5. The sensor according to claim 1, wherein the two or more
photodetectors are selected from the group comprising
phototransistors, photoresistors, photovoltaic cells, phototubes,
photomultiplier tubes, light-to-voltage converters and
light-to-frequency converters.
6. The sensor according to claim 1, wherein the first
filter-photodetector and second filter-photodetector are mounted on
a common substrate.
7. An illumination system comprising: a) a plurality of
light-emitting elements for emitting different ranges of emission
wavelengths of light; b) one or more sensors, each sensor including
two or more photodetectors, each having a detection zone responsive
to a range of wavelengths in the electromagnetic spectrum, wherein
each photodetector generates a sensor parameter representative of
intensity and chromaticity of light incident upon its detection
zone, each sensor including two or more filters, one filter
optically coupled to a first of the two or more photodetectors
thereby forming a first filter-photodetector and a second filter
optically coupled to a second of the two or more photodetectors
thereby forming a second filter-photodetector; the first
filter-photodetector configured to be sensitive to a first
predetermined region of the electromagnetic spectrum, and the
second filter-photodetector configured to be sensitive to a second
predetermined region of the electromagnetic spectrum, wherein the
first predetermined region is complementary to the second
predetermined region, said first filter-photodetector and second
filter-photodetector forming a complementary pair; c) measuring
means interfaced with the one or more sensors for independently
measuring sensor signals from each of the one or more sensors to
enable determination of the intensity and chromaticity of the
light; d) driver means coupled to the plurality of light-emitting
elements and configured to generate a drive signal for each of the
plurality of light-emitting elements; and e) control means coupled
to the driver means and the measuring means, the control means for
individually controlling the intensity of light emission of each of
the plurality of light-emitting elements in response to the sensor
signals.
8. The illumination system according to claim 7, wherein the first
predetermined region and the second predetermined region are
configured to overlap in a defined zone of the electromagnetic
spectrum.
9. The illumination system according to claim 8, wherein spectral
responsivity of the first filter-photodetector monotonically
increases with respect to wavelength within the defined zone and
spectral responsivity of the second filter-photodetector
monotonically decreases with respect to wavelength within the
defined zone.
10. The illumination system according to claim 7, wherein the
control means is configured to use pulse width modulation or pulse
code modulation for controlling activation of the plurality of
light-emitting elements.
11. The illumination system according to claim 10, wherein the
measuring means is configured to measure the sensor signals
simultaneously with the activation of the plurality of
light-emitting elements.
12. The illumination system according to claim 7, wherein the
control means is configured to compare the sensor signals with
optimal sensor signal values, thereby providing a means for
determination of adjusted spectral responsivities of the first
filter-photodetector and the second filter-photodetector.
13. The illumination system according to claim 12, wherein the
control means comprises memory for storing the optimal values in a
look-up table.
14. The illumination system according to claim 12, wherein the
control means comprises memory for storing an analytic equation for
calculation of the optimal values.
15. The illumination system according to claim 7, further
comprising one or more condenser lenses for manipulating the light
emitted by one or more of the plurality of light-emitting
elements.
16. The illumination system according to claim 7, further
comprising a diffuser for blending the light emitted by the
plurality of light-emitting elements.
Description
FIELD OF THE INVENTION
The present invention relates to optical sensing devices, and more
particularly to multicolour chromaticity sensing devices for use
with lighting devices.
BACKGROUND OF THE INVENTION
Light-emitting diodes (LEDs) are semiconductor devices that convert
electrical energy directly into visible light of various colours.
With the advent of high-flux LEDs, luminaires are progressively
being moved from the traditional incandescent or fluorescent lamps
to LEDs for increased reliability, higher luminous efficacy and
lower maintenance costs. LED-based luminaires are increasingly
becoming the architecture of choice in a variety of mainstream
commercial applications such as accent lights, wall washing,
signage, advertising, decorative and display lighting, facade
lighting, and custom lighting, for example.
LEDs are also being used as energy-efficient and long-lived
replacements for cold cathode fluorescent lamps (CCFLs) currently
employed for backlighting of liquid crystal display (LCD) panels
for televisions and computer monitors. Unlike CCFLs which have
relatively broadband spectral power distributions, the narrow
spectral bandwidths of red, green and blue LEDs can be suited for
the corresponding colour filters of LCD panels.
While colour LEDs, for example red, green and blue LEDs, can be
used to generate white light for use in LED-based luminaries and
LCD panel backlighting, the white light's chromaticity is dependent
on the combination of intensities and dominant wavelengths of the
LEDs which are combined to produce white light. These optical
parameters can vary even when the LED drive current is constant,
due to such factors as heat sink thermal constants, changes in
ambient temperature, and LED device aging.
One solution to this problem is to employ optical feedback to
continuously measure the white light intensity and chromaticity and
adjust the drive currents of the LEDs of various colours such that
the intensity and chromaticity of the white light remains
substantially constant. This solution requires a reliable and
relatively inexpensive means of measuring both intensity and
chromaticity.
One approach for measuring intensity and chromaticity relies on
tristimulus colour sensors such as those manufactured by
Hamamatsu.TM. and TAOS.TM.. These tristimulus colour sensors
typically comprise a colourimeter comprising three sensors
(typically silicon photodiodes) whose spectral responsivities are
modified by dyed colour filters to approximate the Commission
Internationale de l'Eclairage.TM. (CIE) red ( x), green ( y), and
blue ( z) colour matching functions of the human visual system, and
wherein the combination of filters with photodetectors represent a
tristimulus colour sensor. The colourimeter thereby determines the
intensity and chromaticity of incident white light by measuring the
sensor output with a suitable electrical device, for example a
current meter. While it can be difficult and expensive to
manufacture suitable filter-photodetector combinations to
approximate the colour matching functions of the human visual
system, tristimulus colour sensors may be used to directly measure
white light intensity and chromaticity. For example, the y colour
matching function is equivalent to the CIE V(.lamda.) spectral
luminous efficiency function for photopic vision, and therefore
represents luminous intensity.
In practice, however, the spectral responsivities of commercial
tristimulus colour sensors such as those manufactured by
Hamamatsu.TM. and TAOS.TM. can only roughly approximate the CIE
colour matching functions. If the dominant wavelengths and spectral
power distributions of the LEDs of various colours (such as red,
green and blue) are fixed and roughly correspond to the peak
wavelength responsivities of the tristimulus colour sensor, the
three outputs of a tristimulus colour sensor can be used to measure
the intensities of the various colours generated by the LEDs. On
the basis of this information, the intensity and chromaticity of
the resultant white light can be approximately calculated.
There are however three complicating factors. First, both the
spectral power distributions of the colour LEDs and the spectral
responsivities of the filter-photodetector combinations overlap, so
there can be optical crosstalk between the three output channels of
the tristimulus colour sensor. For example, the green channel of
the tristimulus colour sensor will respond to radiant flux emitted
by a blue or red LED.
Second, white light generated by red, green, blue, and amber LEDs
is known to have better colour rendering properties than white
light generated by red, green, and blue LEDs. The contribution of
the amber light flux to the white light results in a composite
spectral power distribution that more closely approximates that of
a blackbody light source, which by definition has a CIE colour
rendering index of 100. However, the red and green channels of the
tristimulus colour sensor generally exhibit significant responses
to the amber LEDs. The intensity of the amber LEDs therefore cannot
be determined unless the intensities of light generated by the red
and green LEDs and their contributions to the red and green channel
outputs are known.
Third, even if the spectral power distributions of the colour LEDs
and the spectral responsivities of the filter-photodetector
combinations of the tristimulus colour sensor do not overlap, any
change in the dominant wavelengths of the light produced by the
LEDs can result in changes in the tristimulus sensor output. Even
if the light-emitting sources are wavelength-tunable monochromatic
lasers, the responsivities of the filter-photodetector combinations
typically are not constant with respect to wavelength, and the
tristimulus sensor output will therefore vary as each laser's
wavelength is changed. This problem can be partially alleviated by
using thin-film interference filters that have essentially constant
bandpass characteristics within a specified range of wavelengths.
When used with monochromatic LEDs, these filters can eliminate to
some extent the optical crosstalk between channels of the
tristimulus sensor. However, LEDs used in lighting applications
typically have spectral full width half maximum values of between
15 and 35 nm, so optical crosstalk will typically occur unless the
spectral power distribution of a colour LED is completely within
the wavelength range of its corresponding colour filter. If the
LEDs' spectral power distributions themselves overlap, for example
as occurs with red and amber LEDs, optical crosstalk will be
unavoidable with tristimulus colour sensors.
Another proposed approach is to use a spectroradiometer, wherein
incident white light illuminates a slit and a diffractive element
disperses the polychromatic light onto a linear sensor array whose
photosensitive elements are sequentially measured by a measuring
instrument such as a current meter. To be useful, the spectral
resolution of the spectroradiometer must be better than the
smallest acceptable change in dominant wavelength in order to avoid
perceptible colour shifts in the white light. However, most
spectroradiometer designs require precision optics and a
considerable volume of space that is incompatible with
microelectronic subsystems. Moreover, most of the existing
spectroradiometer designs are typically difficult to fabricate,
especially those based on micromachined moving parts.
Regardless of the spectroradiometer design, the sensor output
typically comprises many different photodetector readings for each
spectral wavelength range of 10 nm or less that are assembled into
a relative spectral power distribution and then analyzed to
determine the relative intensity and dominant wavelength of each
LED. The processing power needed to perform this analysis generally
requires a fast microprocessor, without which, the processing time
may prevent the spectroradiometer from being used for real-time
applications where the input signals change over a period of
milliseconds.
What is clearly needed is a device with the simplicity and
potential ease of manufacture of colourimetric sensors, but which
does not suffer from the problem of varying output with changes in
dominant wavelength. The spectroradiometer approach fails in that
such devices are generally complex and expensive to manufacture,
and they generate an overabundance of data that must be analyzed to
obtain a few significant values, for example LED intensity and
dominant wavelength.
U.S. Pat. No. 4,238,760 to Carr teaches a plurality of photodiodes
that are constructed vertically on a common semiconductor
substrate, whereby each photodetector exhibits spectral
responsivity to different regions of the electromagnetic spectrum.
The photodiode design disclosed by Carr has also been extended to
implement tristrimulus photodiode arrays, such as those disclosed
by Turner et al. in U.S. Pat. No. 6,864,557. A disadvantage of the
photodiode design disclosed by Carr is that it can be difficult to
obtain predetermined and desirable spectral responsivities solely
through the use of semiconductor manufacturing techniques. For
example, the photodiode design disclosed by Carr exhibits broad
spectral responsivities for the blue and red photodiodes. As a
result, the spectral resolution of Carr's photodiodes may be poor,
particularly in the presence of electrical noise.
Therefore there is a need for a new multicolour chromaticity sensor
that is relatively simple, while providing the desired level of
detection.
This background information is provided to reveal information
believed by the applicant to be of possible relevance to the
present invention. No admission is necessarily intended, nor should
be construed, that any of the preceding information constitutes
prior art against the present invention.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a multicolour
chromaticity sensor. In one aspect of the present invention there
is provided a multicolour chromaticity sensor enabling
determination of intensity and chromaticity of light, the sensor
comprising: two or more photodetectors, each having a detection
zone responsive to a range of wavelengths in the electromagnetic
spectrum, wherein each photodetector generates a sensor parameter
representative of intensity and chromaticity of light incident upon
its detection zone; two or more filters, one filter optically
coupled to a first of the two or more photodetectors thereby
forming a first filter-photodetector and a second filter optically
coupled to a second of the two or more photodetectors thereby
forming a second filter-photodetector; and the first
filter-photodetector configured to be sensitive to a first
predetermined region of the electromagnetic spectrum, and the
second filter-photodetector configured to be sensitive to a second
predetermined region of the electromagnetic spectrum, said first
filter-photodetector and second filter-photodetector forming a
complementary pair, wherein the first predetermined region is
complementary to the second predetermined region; wherein a
measuring means interfaced with each of the two or more
photodetectors independently receives a first sensor parameter from
for one of the two or more photodetectors and a second sensor
parameter for a second of the two or more photodetectors, thereby
providing a means to determine the intensity and chromaticity of
the light.
In another aspect of the present invention there is provided an
illumination system comprising: a plurality of light-emitting
elements for emitting different ranges of emission wavelengths of
light; one or more sensors, each sensor including two or more
photodetectors, each having a detection zone responsive to a range
of wavelengths in the electromagnetic spectrum, wherein each
photodetector generates a sensor parameter representative of
intensity and chromaticity of light incident upon its detection
zone, each sensor including two or more filters, one filter
optically coupled to a first of the two or more photodetectors
thereby forming a first filter-photodetector and a second filter
optically coupled to a second of the two or more photodetectors
thereby forming a second filter-photodetector; the first
filter-photodetector configured to be sensitive to a first
predetermined region of the electromagnetic spectrum, and the
second filter-photodetector configured to be sensitive to a second
predetermined region of the electromagnetic spectrum, wherein the
first predetermined region is complementary to the second
predetermined region, said first filter-photodetector and second
filter-photodetector forming a complementary pair; measuring means
interfaced with the one or more sensors for independently measuring
sensor signals from each of the one or more sensors to enable
determination of the intensity and chromaticity of the light;
driver means coupled to the plurality of light-emitting elements
and configured to generate a drive signal for each of the plurality
of light-emitting elements; and control means coupled to the driver
means and the measuring means, the control means for individually
controlling the intensity of light emission of each of the
plurality of light-emitting elements in response to the sensor
signals.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is diagrammatic view of a multicolour chromaticity sensor
according to an embodiment of the present invention.
FIG. 2 is a graph illustrating the responsivities of the filter and
photodetector pair of FIG. 1 over a portion of the electromagnetic
spectrum.
FIG. 3 is diagrammatic view of a multicolour chromaticity sensor
having a plurality of filter-photodetector pairs according to
another embodiment of the invention.
FIG. 4 is a diagrammatic view of an illumination system according
to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
The term "light-emitting element" is used to define any device that
emits radiation in any region or combination of regions of the
electromagnetic spectrum for example, the visible region, infrared
and/or ultraviolet region, when activated by applying a potential
difference across it or passing a current through it, for example.
Therefore a light-emitting element can have monochromatic,
quasi-monochromatic, polychromatic or broadband spectral emission
characteristics. Examples of light-emitting elements include
semiconductor, organic, or polymer/polymeric light-emitting diodes,
optically pumped phosphor coated light-emitting diodes, optically
pumped nano-crystal light-emitting diodes or any other similar
light-emitting devices as would be readily understood by a worker
skilled in the art. Furthermore, the term light-emitting element is
used to define the specific device that emits the radiation, for
example a LED die, and can equally be used to define a combination
of the specific device that emits the radiation together with a
housing or package within which the specific device or devices are
placed.
As used herein, the term "about" refers to a +/-10% variation from
the nominal value. It is to be understood that such a variation is
always included in any given value provided herein, whether or not
it is specifically referred to.
The term "chromaticity" is used to define the perceived colour
impression of light according to standards of the CIE.
The term "colour matching function" is used to define the spectral
tristimulus values per unit wavelength and unit spectral radiant
flux, according to the standards of the CIE.
The term "peak wavelength" is used to define the wavelength at
which the spectral radiant flux per unit wavelength is maximal,
according to the standards of the CIE.
The term "dominant wavelength" is used to define the wavelength of
radiant flux of a single frequency that, when combined in suitable
proportion with the radiant energy of a reference standard, matches
the chromaticity of a perceived light source, according to the
standards of the CIE.
The term "gamut" is used to define the plurality of chromaticity
values that an illumination system is able to achieve.
The term "intensity" is used to define the luminous intensity of a
light source according to standards of the CIE.
The term "sensor" is used to define an optical device having a
measurable sensor parameter in response to a characteristic of
incident light, such as its chromaticity or spectral intensity.
The term "spectral intensity" is used to define the spectral
radiant intensity, according to the standards of the CIE.
The term "spectral power distribution" is used to define the
spectral radiant flux per unit wavelength, according to the
standards of the CIE, over a predefined range of wavelengths.
The term "spectral responsivity" is used to define the responsivity
of a sensor per unit wavelength, over a predefined range of
wavelengths.
The term "spectral transmittance" is used to define the ratio of
transmitted radiant flux to incident radiant flux per unit
wavelength, according to the standards of the CIE, over a
predefined range of wavelengths.
The term "spectral resolution" is used to define the minimum
separation between two different wavelengths in the optical
spectrum as distinguishable by the sensor. This is quantified by
separation .DELTA..lamda., where .lamda. is the measurement
wavelength.
Unless defined otherwise, all technical and scientific terms used
herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs.
The present invention arises from the realization that generation
of white light with a substantially constant intensity and
chromaticity by an illumination system including a combination of
light-emitting elements of different colours is difficult to
achieve due to the variations in the intensities and dominant
wavelengths of the light-emitting elements. These variations are
generally due to ambient temperature, age of the light-emitting
elements, drive current, and various other physical conditions of
the light-emitting elements.
The present invention provides an optical sensor having one or more
filter-photodetector pairs and feedback to monitor the intensity
and chromaticity of the white light generated by the illumination
system and adjust the drive current to the light-emitting elements
in order to maintain substantially constant intensity and
chromaticity of the output white light irrespective of variations
with respect to the light-emitting elements. According to the
present invention, filter-photodetectors are configured into pairs
thereof, wherein a first filter-photodetector of a pair is
configured and arranged so as to be sensitive to a predetermined
region of the electromagnetic spectrum, while a second
filter-photodetector of the pair is configured and arranged to be
sensitive to a substantially complementary region of the
electromagnetic spectrum. The spectral responsivity of the first
filter-photodetector and the second filter-photodetector overlap in
a predetermined region of the electromagnetic spectrum.
Furthermore, the spectral responsivity of the first
filter-photodetector is configured to substantially monotonically
increase with respect to wavelength within said predetermined
region, while the spectral responsivity of the second
filter-photodetector is configured to substantially monotonically
decrease with respect to wavelength within said predetermined
region.
Multicolour Chromaticity Sensor
Reference is now made to FIG. 1 which illustrates a multicolour
chromaticity sensor according to an embodiment of the present
invention. The sensor 10 includes colour filters 12a, 12b optically
coupled to photodetectors 14a, 14b, each having a detection zone
16a, 16b for determining a sensor parameter in relation to the
intensity and chromaticity of an incident light 11. The outputs of
photodetectors 14a, 14b are interfaced to a measuring device 18
which independently evaluates the sensor parameters from the
photodetectors 14a, 14b.
The colour filters 12a, 12b may be thin-film interference filters
or transmissive dyed colour filters, wherein each filter provides a
different band-pass spectral transmittance. Filters employing
photonic crystals having resonance abnormalities or guided-mode
resonance filters may also be used. Alternatively, tunable liquid
crystal Lyot band pass filters, a single layer of liquid crystal
material having a fairly wide bandpass spectral transmittance,
plasmon filters or other types of optical filters as contemplated
by those skilled in the art may also be employed.
The photodetectors 14a, 14b can be light-to-current converters
comprising a photodiode and a current amplifier and these elements
of the photodetectors may be configured for example on a single
chip. However, the photodetectors 14a, 14b may comprise other
similar light detection devices as is known to those skilled in the
art, including but not limited to phototransistors, photoresistors,
photovoltaic cells, phototubes, photomultiplier tubes or other
formats of light-to-voltage converters or light-to-frequency
converters. The photodetectors 14a, 14b may include lens elements
(not shown) in front of the detection zones 16a, 16b thereof in
order to enhance the amount of light that is absorbed by the
detection zones 16a, 16b. The output of the photodetectors 14a, 14b
is typically in the form of an electric signal proportional to the
intensity of the light incident on the detection zones, 16a and
16b.
In one embodiment of the present invention, the first filter 12a
and photodetector 14a of a pair and the second filter 12b and
photodetector 14b of the pair can be mounted on a common substrate.
Since the efficiency of the photodetectors 14a, 14b is dependent
upon the operating temperature of the photodetectors 14a, 14b, both
photodetectors 14a, 14b can be mounted on an isothermal region of
the common substrate. Accordingly, while the absolute spectral
responsivities of the photodetectors 14a, 14b may change with
ambient temperature, their relative spectral responsivities can
remain effectively unchanged.
As previously mentioned, in the presently described embodiment, the
colour filters 12a, 12b are optically coupled to photodetectors
14a, 14b. In one embodiment, the first filter 12a and photodetector
14a of the pair can be sufficiently spaced from the second filter
12b and photodetector 14b of the pair to avoid optical cross-talk
therebetween. In an alternate embodiment, the colour filters 12a,
12b could be in spaced relationship with the photodetectors 14a,
14b. The colour filters 12a, 12b and photodetectors 14a, 14b can be
configured and arranged along an axis that is perpendicular or at
an angle with respect to the axis of the incident light 11.
In the presently described embodiment, the first filter 12a and
photodetector 14a of the pair is configured and arranged so as to
be sensitive to a predetermined region of the electromagnetic
spectrum, for example blue light, while the second filter 12b and
photodetector 14b of the pair is configured and arranged to be
sensitive to a substantially complementary region of the
electromagnetic spectrum, such as red light. The spectral
responsitivity of the first filter 12a and photodetector 14a and
the second filter 12b and photodetector 14b overlaps in a
predetermined region of the electromagnetic spectrum, for instance
in the green light region. Furthermore, the spectral responsitivity
of the first filter 12a and photodetector 14a substantially
monotonically increases with respect to wavelength within said
predetermined region, while the spectral responsitivity of the
second filter 12b and photodetector 14b substantially monotonically
decreases with respect to wavelength within said predetermined
region.
In one embodiment of the present invention, if the incident light
11 has a relative spectral power distribution I(.lamda.) and the
spectral responsivities of the pair of first and second filters
12a, 12b and photodetectors 14a, 14b are respectively
R.sub.a(.lamda.) and R.sub.b(.lamda.), the outputs of said
filter-photodetector pairs are respectively
V.sub.a=.intg..sub..lamda.I(.lamda.)R.sub.a(.lamda.)d.lamda. and
V.sub.b=.intg..sub..lamda.I(.lamda.)R.sub.b(.lamda.)d.lamda.. If
the intensity I of the incident light 11 is varied while the
relative spectral power distribution I(.lamda.) remains constant,
the outputs of the pair of first and second filters 12a, 12b and
photodetectors 14a, 14b and the quotient V.sub.a/V.sub.b thereof
also remain substantially constant. If, on the other hand, the
relative spectral power distribution I(.lamda.) of the incident
light 11 varies, the output of the pair of filter-photodetectors
and the quotient thereof will also vary. If the change in the
relative spectral power distribution I(.lamda.) is characterized by
a shift in peak wavelength, simultaneous changes in intensity I and
the relative spectral power distribution I(.lamda.) can be
mathematically separable. This relationship between the intensity
and relative spectral power distribution of the incident light and
the outputs of the filter-photodetector pairs may therefore be used
to calibrate and subsequently independently determine the intensity
and chromaticity of white light generated by an illumination system
comprising a plurality of light-emitting elements.
FIG. 2 illustrates the spectral responsivity of the first filter
12a and photodetector 14a of the pair compared to the spectral
responsivity of the second filter 12b and photodetector 14b of the
pair according to one embodiment of the present invention. As can
be readily observed, the effect of the pair of filters 12a, 12b and
photodetectors 14a, 14b is that if the detected light exhibits a
peak spectral intensity within the predetermined region, the
combined outputs of the pair of filters 12a, 12b and photodetectors
14a, 14b will be substantially independent of the peak wavelength
of the relative spectral power distribution. As an example,
consider incident light 11 to be monochromatic radiation from a
tunable-wavelength laser with an initial wavelength within said
predetermined region of the spectrum. If the intensity I of the
incident light 11 is increased or decreased, the outputs of
filter-photodetector pair 12a, 14a and 12b, 14b will increase or
decrease proportionately according to I=A+B, where A is the output
of filter-photodetector pair 12a, 14a, and B is the output of
filter-detector 12b, 14b.
In one embodiment of the present invention, if the peak wavelength
of the relative spectral power distribution increases, the output
of the first filter 12a and photodetector 14a will increase, while
the output of the second filter 12b and photodetector 14b will
decrease. As an example, if wavelength .lamda. of incident light 11
from a monochromatic light source is increased, the output of
filter-photodetector 12a, 14a will decrease while the output of
filter-photodetector 12b, 14b will increase. Conversely, if
wavelength .lamda. of incident light 11 is decreased, the output of
filter-photodetector 12a, 14a will increase while the output of
filter-detector 12b, 14b will decrease. For example, assuming that
wavelength .lamda. remains within the predetermined region of the
spectrum, said wavelength will be proportional to B+(1-A)/(A+B),
relative to the minimum wavelength .lamda..sub.min of said
predetermined region of the spectrum. Similarly, for example the
wavelength can be proportional to quotient A/B, although in a
typically less linear manner.
In one embodiment of the present invention, the sensor outputs can
be measured by the measuring device 18, which can be a current
meter combined with additional circuitry for conditioning of the
signal from the current meter, as would be know to a worker skilled
in the art. The measuring device 18 receives the respective outputs
of the pair of filters 12a, 12b and photodetectors 14a, 14b and
determines the relationships of the outputs.
For applications using for example substantially high-frequency
pulse width or pulse code modulation for controlling the activation
of the light-emitting elements, the intensity of the incident light
11 can vary rapidly, and therefore it may be necessary to measure
the output of the pair of filters 12a, 12b and photodetectors 14a,
14b simultaneously with the activation of the light-emitting
elements, in order to avoid discrepancies in the measured output
due to the time-variations of incident light 11. Accordingly, the
measuring device 18 can include additional circuitry (not shown)
such as parallel flash analog-to-digital converters or
sample-and-hold circuitry to simultaneously measure the output of
the pair of filters 12a, 12b and photodetectors 14a, 14b with the
activation of the light-emitting elements.
In one embodiment of the present invention, in operation the
physical elements employed to implement the filters 12a, 12b and
the photodetectors 14a, 14b may not exhibit perfect or near perfect
behaviour. It therefore may be difficult to obtain constant slope
attenuation with respect to wavelength for physically realizable
spectral responsivities as illustrated in FIG. 2. As a result, the
relationship between the intensity and chromaticity of incident
light 11 and the outputs of the pair of filters 12a, 12b and
photodetectors 14a, 14b may become nonlinear. In order to account
for these possible nonlinearities associated with the physically
realizable components for the pair of filters 12a, 12b and
photodetectors 14a, 14b, in one embodiment of the present invention
the outputs of the pair of filters 12a, 12b and photodetectors 14a,
14b can be compared with a lookup table containing optimal values
and these outputs can be re-evaluated by analytic approximation in
an attempt to linearize the spectral responsivities for the pair of
filters 12a, 12b and photodetectors 14a, 14b. Similarly, where the
incident light 11 has a variable spectral power distribution such
as occurs when high-flux LEDs are used, for example when the
spectral bandwidth and peak wavelength changes with increasing or
decreasing drive current, changes in ambient temperature or peak
wavelength variations due to colour binning of the LEDs during
manufacture, experimental measurement or computer simulated
measurements may be required to determine approximating analytic
equation coefficients or numerical lookup tables as disclosed in
for example co-pending U.S. patent application Ser. No. 10/897,990
"Control System for an Illumination Device Incorporating Discrete
Light Sources", herein incorporated by reference.
As an example, a blue LED based on indium-gallium-nitride (InGaN)
alloys may exhibit a peak wavelength shift as the drive current is
reduced from full rated maximum current. For example, in co-pending
U.S. Provisional Patent Application No. 60/772,458 "Light Source
Intensity Control System and Method", herein incorporated by
reference, a linear combination of two Gaussian functions with
different centre wavelengths may be used to analytically model the
LED spectral power distribution. This format of analytic model may
be usefully employed rather than numerical lookup tables which may
be memory-intensive.
As will be appreciated by those skilled in the art, the intensity
and peak wavelength of a light source cannot be used to directly
determine the chromaticity of the emitted light. However, for light
sources such as LED-based illumination systems with red, green, and
blue LEDs, the relative spectral power distribution comprises a
plurality of peak wavelengths with relatively narrow spectral
bandwidths. If changes in the intensity and peak wavelengths of
each LED colour are measured with a complementary pair of filters
and photodetectors, the corresponding change in the combined light
intensity and chromaticity for the light source can be
determined.
Referring to FIG. 3, a multicolour chromaticity sensor 100 in
accordance with another embodiment of the present invention is
illustrated. The sensor 100 includes a plurality of colour filters
12a . . . 12n optically coupled to photodetectors 14a . . . 14n,
respectively, each having a detection zone 16a . . . 16n associated
therewith for determining a sensor parameter in relation to the
intensity and chromaticity of incident light 110. This incident
light 110 has a gamut that is the combination of light of various
wavelengths generated by the light-emitting elements (not shown),
whereby each light-emitting element produces light having a
specific range of wavelengths in the electromagnetic spectrum. In
the diagrammatic representation of sensor 100 in FIG. 3, only the
colour filters 12a, 12b, 12m and 12n, and photodetectors 14a, 14b,
14m and 14n, are shown. It would be readily understood that any
number of filter-photodetector pairs can be used in the multicolour
chromaticity sensor according to the present invention.
The first filter 12a and photodetector 14a form a complementary
pair with the second filter 12b and photodetector 14b. In a similar
fashion, the mth filter 12m and photodetector 14m form a
complementary pair with the nth filter 12n and photodetector 14n.
Each filter and photodetector of a pair is configured and arranged
so as to be sensitive to a predetermined region of the
electromagnetic spectrum, while the corresponding complementary the
filter and photodetector of the pair is configured and arranged to
be sensitive to a substantially complementary region of the
electromagnetic spectrum. The spectral responsivities of the mth
filter and photodetector and of its complementary filter and
photodetector can overlap in a predetermined region of the
electromagnetic spectrum. Moreover, the spectral responsitivity of
the mth filter and photodetector substantially monotonically
increases with respect to wavelength within said predetermined
region, while the spectral responsitivity of the complementary
filter and photodetector substantially monotonically decreases with
respect to wavelength within said predetermined region.
In one embodiment of the present invention, the outputs of
photodetectors 14a . . . 14n are interfaced to a measuring device
180 which can independently evaluate the sensor parameters from the
photodetectors 14a . . . 14n. The quotient of the output of the mth
filter and photodetector of the pair divided by the output of the
complementary filter and photodetector of the pair can be
proportional to the peak wavelength. This quotient can be measured
by the measuring device 180 and can serve to resolve the intensity
and chromaticity of the incident light 110.
In one embodiment of the present invention and with reference to
FIG. 3, for a given number n of light-emitting elements, the number
of filters 12a . . . 12n and the number of photodetectors 14a . . .
14n required to implement the sensor 100 may be described by
expression (1), as follows: v(k)=2.times.k where: v(k) is the
number of filters 12a . . . 12n or the number of photodetectors 14a
. . . 14n; and k is the number of light-emitting elements of
different wavelength ranges. Illumination System with Multicolour
Chromaticity Sensor
Reference is now made to FIG. 4, which shows an illumination system
according to an embodiment of the present invention. The
illumination system includes a plurality of light-emitting elements
202, 204 and 206 emitting electromagnetic radiation at different
peak wavelengths. In the presently described embodiment of the
invention, the light-emitting elements are LEDs, however other
types of light-emitting elements as is known to those skilled in
the art can also be used. The light-emitting elements 202, 204 and
206 are configured and arranged in a red array, a green array, and
a blue array, respectively.
In one embodiment a condenser lens 222 or the like can be provided
to enhance the optical output of the red array 202, for example.
Like condenser lenses 224 and 226 or alternate optical elements can
be provided for the blue and green arrays of light-emitting
elements.
The light emitted from the red, green, and blue arrays, which can
be emitted either sequentially or simultaneously, can provide a
steady optical throughput of white light 110 composed of the
combination of the red, green and blue light colours. In one
embodiment, an optical diffuser 300 is provided to further
spatially blend the constituent red, green and blue light colours,
thereby improving the uniformity of the colour mixing and thereby
generating white light 110 of a substantially uniform
chromaticity.
With further reference to FIG. 4, the first filter 122 and
photodetector 142 form a complementary pair with the second filter
123 and photodetector 143. In a similar fashion, third filter 124
and photodetector 144 form a complementary pair with the fourth
filter 125 and photodetector 145, and the fifth filter 126 and
photodetector 146 form a complementary pair with the 6th filter 127
and photodetector 147. Each of the filters 122, 124 and 126, and
photodetectors 142, 144 and 146, respectively, is configured and
arranged so as to be sensitive to a predetermined region of the
electromagnetic spectrum, while their corresponding complementary
filters 123, 125 and 127, and photodetectors 143, 145 and 147, are
configured and arranged to be sensitive to a respectively
substantially complementary region of the electromagnetic spectrum.
As a result, the spectral responsivities of filters 122, 124 and
126, and photodetectors 142, 144 and 146 and their respective
complementary filter 123, 125 and 127, and photodetector 143, 145
and 147 pairs overlap in a multiplicity of predetermined regions of
the electromagnetic spectrum. In addition, the spectral
responsitivity of filters 122, 124 and 126, and photodetectors 142,
144 and 146 substantially monotonically increases with respect to
wavelength within each said predetermined region, while the
spectral responsitivity of their respective complementary filter
123, 125 and 127, and photodetector 143, 145 and 147 substantially
monotonically decreases with respect to wavelength within each said
predetermined region, and wherein each said predetermined region
includes the expected variation in peak wavelength of
light-emitting elements 202, 204 and 206 respectively.
In one embodiment of the present invention, the outputs of
photodetectors 142 to 147 are interfaced to the measuring device
280 which independently evaluates the sensor parameters from the
photodetectors 142 to 147. The quotient of the output of filters
122, 124 and 126, and photodetectors 142, 144 and 146 divided by
the output of their respective corresponding filter 123, 125 and
127, and photodetector 143, 145 and 147 can be proportional to the
peak wavelength of the red, green and blue lights, respectively.
Each quotient can be measured by the measuring device 280 and can
serve to resolve the intensity and chromaticity of the incident
light 110.
In one embodiment of the present invention, a driver circuit 400
module coupled to the light-emitting elements 202, 204 and 206 can
be configured to generate a drive signal for independently driving
the light-emitting elements 202, 204 and 206. A controller 500 can
communicate with the driver circuit 400. The controller 500 can be
implemented by a microprocessor or the like and can control the
amount of current supplied to each light-emitting elements 202, 204
and 206. In one embodiment of the present invention the control of
the current supplied to the light-emitting elements can be
performed using pulse width modulation, pulse code modulation or
other method as would be readily understood by a worker skilled in
the art.
In one embodiment of the present invention, the controller 500 can
interface with the measuring device 280 in a feedback loop
configuration. The feedback loop configuration can allow the
controller 500 to constantly monitor the intensity and chromaticity
of the incident light 110 based on the sensor parameters determined
by the measuring device 280, and determine the amount of current to
be supplied to each of the light-emitting elements 202, 204 and 206
in order to maintain constant intensity and chromaticity of the
generated incident light 110.
It is obvious that the foregoing embodiments of the invention are
exemplary and can be varied in many ways. Such present or future
variations are not to be regarded as a departure from the spirit
and scope of the invention, and all such modifications as would be
obvious to one skilled in the art are intended to be included
within the scope of the following claims.
* * * * *
References